ARCHIVED REPORTS_RESULTS OF THE UPDATED HUMAN HEALTH RISK SCREENING EVALUATION

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GEOMATRIX Mr. Robert Mihalovich Chevron Environmental Management Company July 20, 2001 Page 2 laboratory reporting limits in soil samples. TPH quantified as diesel (TPHd) was detected in grab groundwater samples in May 2000. Analytical results from the October 2000 sampling event indicated that the following constituents were detected in grab groundwater samples: TPHd, fluorene (0.3 micrograms per liter [µg/1]), phenanthrene (0.2 gg/1), toluene (0.7 µg/1), and xylene (1.5 gg/1). Phenanthrene, toluene, and xylenes were not detected above the labo- ratory reporting limits in previous groundwater samples and fluorene was previously detected at the same concentration. In accordance with Cal-EPA PEA guidance, the updated screening health risks presented in this letter are based on the maximum detected concentration for each constituent. Thus, the three newly detected constituents (phenanthrene, toluene, and xylene) were added and evaluated as chemicals of potential concern, and the screening human health risks were recalculated. TPHd was not evaluated because toxicity health criteria do not exist for TPH mixtures. Incorporating the May and October 2000 analytical data, the theoretical excess lifetime cancer risk and the noncancer hazard index for a future adult and child resident did not change be- cause the concentrations of the newly detected constituents did not contribute significantly to the overall risks. The lifetime excess cancer risk and the noncancer hazard index are 1x10-6 and 0.65, respectively. The lifetime excess cancer risk is at the lower end of the acceptable • risk range of 1x10-6 to 1x10 used by Cal-EPA and U.S. EPA for sites where remediation is considered. The hazard index is less than 1, indicating that potential exposures to petroleum constituents at the site should not result in an adverse noncarcinogenic health effect. The up- dated revised risk calculations are presented in Attachment 3. In summary, based on the new analytical data, the screening risk evaluation indicates that potential exposures to petroleum constituents in soil and groundwater should not pose an unacceptable health risk to hypothetical future residential receptors under the conditions evaluated. In addition, based on the conservative nature of the PEA evaluation, the presence of residual petroleum constituents in soil and groundwater should not pose an unacceptable human health risk to future commercial and construction workers at the site. If you have any questions or if we can be of further assistance,please do not hesitate to call. Sincerely yours, GEOMATRIX CONSULTANTS, INC. Robert H. CheunG R. Foote R.G. g �'Y Senior Toxicologist Senior Geologist ncc/grf/pp \\sf3\deptdata\Project\7000s\7243\update risk Itr-Burns-Ridgeway.doc Attachments 1 through 3 cc: Penny Fottrell, Geomatrix Consultants, Inc.